For a fascinating review of the whole legal tangle, see G. Alon, “The Levitical Uncleanness of Gentiles,” Jews and Judaism in the Classical World (Jerusalem: The Magnes Press, 1977), pp. 146–189; his discussion of Niddah and Gentile blood is found on p. 154f. For more information on the same issue and tractate, see Sanders, Jewish Law from Jesus to the Mishnah, p. 156.